Abstract

Archean gneisses of the Nain Province in the Nain area, Labrador, comprise narrow septa between intrusions of the Mesoproterozoic Nain Plutonic Suite. This region occupies a critical position between the northern (Saglek) and southern (Hopedale) blocks of the Nain Province, which experienced distinct thermotectonic histories until the late Archean. Field and geochronological evidence are indicative of a strong late Archean thermotectonic overprint that is absent from most of the remainder of the Nain Province. Archean gneisses were intruded by granites and mafic dykes at 2578 ± 3 and, respectively, and subsequently subjected to amphibolite- to granulite-facies regional metamorphism and ductile shearing at ca. 2550 Ma; granite veins and dykes related to the Nain Plutonic Suite were emplaced at ca. 1310 Ma. The Archean events are interpreted to represent the juxtapositioning and final docking of Saglek and Hopedale blocks to form a single, stable cratonic mass during the late Archean. Corroborative evidence indicates that the collisional boundary zone could extend at least 200 km to the north and 150 km to the south of Nain. This late Archean junction may have subsequently been exploited by several Paleoproterozoic granitic intrusions and some members of the Mesoproterozoic Nain Plutonic Suite.

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